Recently, a nonaqueous electrolyte battery such as a lithium-ion secondary battery has been developed as a battery having a high energy density. The nonaqueous electrolyte battery is expected to be used as a power source for vehicles such as hybrid vehicles or electric cars or a large-sized power source for electricity storage. Particularly, for use in vehicles, the nonaqueous electrolyte battery is desired to have other performances such as rapid charge and discharge performances and long-term reliability. A nonaqueous electrolyte battery enabling rapid charge and discharge not only remarkably shortens the charging time but also makes it possible to improve performances related to the motive force of a hybrid vehicle and to efficiently recover regenerative energy.
In order to enable rapid charge and discharge, it is necessary for electrons and lithium ions to be able to migrate rapidly between the positive electrode and the negative electrode. When a battery using a carbon based material in the negative electrode undergoes repeated rapid charge and/or discharge, dendrite precipitation of metal lithium occurs on the electrode. Dendrites cause internal short circuits, which can lead to heat generation and fires.
In light of this, a battery using a metal composite oxide as a negative electrode active material in place of a carbonaceous material has been developed. Particularly, in a battery using titanium oxide as the negative electrode active material, rapid charge and discharge can be performed stably. Such a battery also has a longer life than those using a carbonaceous material.
However, a battery using titanium oxide has a problem in that the battery has low energy density due to the reason described below.
At first, titanium oxide has a higher (nobler) potential relative to metal lithium than that of the carbonaceous material. For example, a lithium-absorption and release potential of titanium oxide is 1.5 V based on metal lithium. The potential of titanium oxide is due to the redox reaction between Ti3+ and Ti4+ when lithium is electrochemically absorbed and released. Therefore, it is limited electrochemically. Further, there is the fact that rapid absorption and release of lithium ions can be stably performed at an electrode potential as high as about 1.5 V. Therefore, it is substantially difficult to drop the potential of the electrode to improve energy density.
Further, titanium oxide has a lower capacity per weight. As to the capacity of the battery per unit weight, the theoretical capacity of a lithium-titanium composite oxide such as L4Ti5O12 is about 175 mAh/g. On the other hand, the theoretical capacity of a general graphite-based electrode material is 372 mAh/g. Therefore, the capacity density of titanium oxide is significantly lower than that of the carbon-based negative electrode. This is due to a reduction in substantial capacity because there are only a small number of lithium-absorption sites in the crystal structure and lithium tends to be stabilized in the structure.
In view of such circumstances, a new electrode material containing Ti and Nb has been examined. Particularly, a composite oxide represented by TiNb2O7 has been a focus of attention. This is because during the Li-absorption into this compound, charge compensation, in which Ti changes from tetravalence to trivalence and Nb changes pentavalence to trivalence, takes place, thereby the compound has a theoretical capacity of 387 mAh/g, and can exhibit a high capacity.